1. Field of the Invention
This invention relates to the recovery of vicinal glycol esters from mixtures containing the same, and more specifically to recovery of ethylene glycol esters or propylene glycol esters from mixtures with high- and low-boiling organo-halogen impurities.
2. Description of the Prior Art
Ethylene glycol and 1,2-propylene glycol (hereinafter referred to as propylene glycol) are chemicals of acknowledged commercial importance. Ethylene glycol is used, for example, in the preparation of antifreeze compositions and in the manufacture of polyester fibers. Ethylene glycol manufacturing processes of commercial interest have generally been based upon ethylene oxide as a raw material. Recently, however, processes have been developed which make it possible to produce ethylene glycol and propylene glycol without the necessity for the intermediate manufacture of the epoxide. These processes employ the liquid phase reaction of the olefin, a carboxylic acid and molecular oxygen, in the presence of catalysts. For example, U.S. Pat. No. 3,770,813 employs a conjoint catalyst system involving specific metals (or organic bases) in conjunction with iodine. Other processes such as those of U.S. Pat. No. 3,689,535 and Canadian Pat. No. 888,749 employ certain variable valent cations together with chlorine or bromine (or their compounds) as catalysts. Yet other processes employ noble-metal based systems and alkali metal halides in conjunction with redox systems (see U.S. Pat. No. 3,262,969). These processes, however, result in the production of by-products in addition to the desired vicinal glycol ester products, and these by-products include organo-halogen compounds as well as ester precursors.
The presence of more than minor amounts of such organo-halogen compound impurities in the vicinal glycol products can frequently interfere with their utilization. For example, it is known that in the case of ethylene glycol diacetate, the presence of more than 100 to 200 ppm of organo-halogen compounds in this material interferes with its pyrolysis to form vinyl acetate (see U.S. Pat. No. 3,804,887). Where it is desired to hydrolyze such esters to produce ethylene glycol or propylene glycol, the presence of organo-halogen compound impurities unfavorably affects the hydrolysis since the halogen compounds have a tendency to contaminate the glycol, and their presence introduces corrosion problems, often necessitating employment of more expensive materials of construction than would otherwise be required.
While it is possible to efficiently remove substantially all of the organo-halogen compound impurities by contacting the impure esters containing these impurities with compounds of selected metals, (see, U.S. Pat. No. 3,884,965), it would be advantageous to recover the desired esters by distillation and recycle the separated by-products and unreacted starting materials.